Senior Investigator James Newell of the state police said Nguyen was charged with offering a false instrument for filing.
Nguyen is also facing federal charges for allegedly lying about the purchase of the semiautomatic .223-caliber Bushmaster rifle, equipped with a combat-style flash suppressor, and the .12-gauge Mossberg shotgun used in Monday's slayings of two first responders and the wounding of three others.
Nguyen is not connected to a .38-caliber Smith & Wesson pistol also recovered.
As as a convicted felon, Spengler was banned from owning guns. He served 17 years in state prison for killing his grandmother with a hammer in 1980. Nguyen lived next door to Spengler for about five years, in the house where he killed his grandmother.
Police interviewed Nguyen late on Christmas Eve and she told officers she had bought the guns for personal protection and that they were stolen from her vehicle, but she never reported them stolen.
On Christmas, Nguyen texted an off-duty Monroe County Sheriff's deputy and referenced the killings, the Associated Press says. Later, police said, she called the deputy and admitted she bought the guns for Spengler.
Nguyen's lawyer, Dave Palmiere, said Friday that his client "purchased the weapons legally, and they were stolen," but that she doesn't recall whether she reported the theft to police.
Several neighbors described Nguyen's home in Greece, west of Rochester, as quiet, but said there was constant traffic in and out. There were five or six cars parked in the driveway Friday afternoon.
Earlier Friday, Welsher's son said the weapon actually was stolen from Nguyen's car.
In the doorway of a home in Greece, Welsher said her lawyer advised her not to speak to the media. "All I can say is, I'm being set up," she said.
In a voice message recorded earlier Friday, Welsher told a Democrat and Chronicle reporter she wanted to talk.
"This is nuts," she said in the recording. "I never supplied this man with nothing. My daughter never supplied him with anything. He's setting us up."
"She said she wanted to go up there before there was an arrest," Steven Nguyen said.
Law enforcement sources said Thursday that an arrest in the case was imminent.
"I think he had a crush on her," the son said.